1. Field of the Invention
This invention relates generally to a high temperature ceramic strain gage. The gage is produced from indium-tin-oxide and can function at temperatures in excess of 1500° C.
2. Description of the Prior Art
The accurate measurement of both static and dynamic strain, at elevated temperatures is frequently required to determine the instabilities and life-times of various structural systems, and in particular, advanced aerospace propulsion systems. Conventional strain gages are typically applied to both stationary and rotating components for this purpose but are usually limited in scope due to their intrusive nature, severe temperature limitations and difficulties in bonding.
Thin film strain sensors are particularly attractive in the gas turbine engine environment since they do not adversely effect the gas flow over the surface of a component and do not require adhesive or cements for bonding purposes. Typically, thin film strain gages are deposited directly onto the surface of a component nickel based superalloy or other high temperature substrate by rf sputtering or other known thin film deposition technology and as a result are in direct communication with the surface being deformed. In general, the piezo-resistive response or gage factor (g), of a strain gage is the finite resistance change of the sensing element when subjected to a strain and can result from (a) changes in dimension of the active strain element and/or (b) changes in the resistivity (p) of the active strain element. Further, the active strain elements used in a high temperature static strain gage, must exhibit a relatively low temperature co-efficient of resistance (TCR) and drift rate (DR) so that the thermally induced apparent strain is negligible compared to the actual mechanical applied strain.
One material of choice for high temperature thin film strain gages is a wide band semiconductor, e.g. indium-tin oxide (ITO), due to its excellent electrical and chemical stability and its relatively large gage factor at high temperature. When used alone is usually limited by relatively high TCRs as is the case for many intrinsic semiconductors. However, as disclosed herein the TCR of a self-compensated ITO strain sensor can be reduced using a metal, e.g. Pt as a thin film resistor placed in series with the active ITO strain element.
Aerospace propulsion systems operate at temperatures in excess of 1500° C. Thin film strain gages are used to monitor the structural integrity of components employed in these systems. The high temperature stability and piezoresistive properties depend to a large extent on the thickness of the strain elements.
As the operating temperature of gas turbine engines is increased and new materials are developed to meet these new challenges, there is a need to assess the structural behavior of components in these harsh environments, so that structural models can be validated and newly developed materials can be monitored during actual engine operation. Thin film sensors are ideally suited to make measurements of operational turbine conditions since they have negligible mass and thus, minimal impact on vibration patterns. The sensors are non-intrusive in that the gage thickness is considerably less than the gas phase boundary layer thickness and thus, the gas flow path through the engine will not be adversely affected by these sensors. Not only are these sensors ideally suited for in-situ strain measurement where high gas velocities are encountered, but these strain gages have excellent adhesion and similar thermal expansion coefficients to most oxides used for electrical isolation.